A traditionally advocated coil for wireless power transmission is configured by connecting a right-handed coil and a left-handed coil having the same shape to each other. The amplitude of a leakage magnetic field is proportional to the amplitude of a current flowing in the coil and proportional to the number of windings of the coil. The phase of the leakage magnetic field corresponds to the phase of the current flowing in the coil. As a result, the leakage magnetic field generated from the right-handed coil and the leakage magnetic field generated from the left-handed coil are offset by each other as they have the same amplitude and inverted phases that are inverted with respect to each other, so that the resultant leakage magnetic field after combination can be reduced.
According to this configuration, when the positions of these coils are spaced away from each other in view of avoidance of the impacts of the mutual inductance of the right-handed coil and the left-handed coil, then the wired connection between the two coils is extended, causing increase in the loss, which is a problem found in the state of the art.
As a mode of power transmission that does not rely on the wired connection between the coils, a known method performs power transmission in a wireless manner between multiple resonators each including a coil. As one example, a method is known according to which a plurality of resonators are arranged in an autobus, and wireless power transmission is performed from a plurality of resonators arranged on the ground side to the resonators of the autobus. With regard to the timing at which the wireless power transmission is to take place, the power transmission may be performed at the time of entry and/or exit of passengers while the autobus is stopped. In this context, autobuses may include a kneeling function according to which, taking into account utilization of an autobus by passengers using a wheelchair, the vehicle body is made to be tilted so as to lower the height of the entrance/exit of the autobus. When the wireless power transmission takes place during the stoppage of the autobus with its vehicle body tilted by the kneeling operation, the distances between the resonators of the vehicle body and the corresponding resonators of the ground side may vary depending on the respective resonator pairs. In this case, differences may be created in the coupling coefficients between the resonators, and the differences in the currents flowing in the individual resonator pairs take values that are significantly different from each other. As a result, mere adjustment of the current phase differences of the resonators of the power transmission side may make it difficult or impossible to reduce the leakage magnetic field, which is a problem found in the state of the art.